A cutting tool material wherein a sintered body consisting of more than 70 vol % of diamond and a metal chiefly comprising Co as a bonding material is bonded onto a cemented carbide substrate is commercially available. The said tool blank is commonly used in a cutting tool for processing Al alloys containing a large amount of Si, Cu alloys, etc. despite the high price thereof.
The said tool blank is not only far better in respect to wear resistance than that of the conventional cemented carbide but also sufficiently tough against impact as compared with a tool blank produced from a single crystal natural diamond. The said cutting tool blank, however, has a disadvantage in that the processed surfaces of nonferrous metals and the like cut by said tool blank have greater roughness compared with those processed by the cutting tool of single crystal natural diamond, thereby rendering it impossible to obtain a beautifully finished surface like a mirror. Moreover, particularly when a small and thin workpiece, such as a part of a watch and the like, is processed, the said tool blank has a high cutting resistance, whereby the workpiece is liable to be deformed out of dimensional precision.